|Grade||Level of Evidence|
|A||Multiple double-blind, controlled clinical trials.|
|B||1 double-blind, controlled clinical trial.|
|C||At least 1 controlled or comparative clinical trial.|
|D||Uncontrolled, observational, animal or in-vitro studies only.|
|Grade||Effect||Size of Effect||Comments|
Evidence from different studies are at odds as to whether glycerin improves atopic dermatitis.
Moisturizes the skin by drawing water from the dermis into the epidermis, via modulating water channels known as aquaporins.
Reduced the size of skin lesions in rats and reduced epithelial healing time in human patients who had undergone laser resurfacing.
Visibly smooths the skin for more than 24 hours after application.
May reduce transepidermal water loss and promote barrier repair in damaged skin, though the evidence is conflicting.
Appears to increase the hysteresis and arguably the distensibility of the skin.
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Table of contents:
Glycerin or glycerol is a simple polyol, an alcohol with multiple hydroxyl groups. It is synthesized in the pilosebaceous unit and exists in the stratum corneum of the skin as part of the natural moisturizing factor (NMF), a natural mixture of amino acids, lactates, urea and electrolytes that helps the stratum corneum retain water.
Glycerin appears to take at least 30 minutes to penetrate the epidermis, according to a study on pig skin. It accumulates in the entire thickness of the stratum corneum, creating a reservoir within the lipid bilayers without disrupting its liquid crystallinity or lamellar structure.
3. Effects on the skin
3.1 Increased skin hydration
Glycerin has 3 hydroxyl groups, allowing it to attract and bind water molecules through the formation of strong hydrogen bonds. It is considered a humectant, a class of moisturizer that hydrates the skin by enhancing water absorption from the dermis into the epidermis. On the flip side, this can lead to excessive water loss through evaporation, which is why glycerin is always combined with occlusives that prevent water loss.
Among moisturizing ingredients, glycerin is unique because it can produce moisturizing effects in the skin even after it is no longer present. It does this by modulating water channels called aquaporins, especially aquaporin-3, which transports both water and glycerin and is abundantly expressed in the epidermis. Mice deficient in aquaporin-3 have reduced stratum corneum hydration, skin elasticity and impaired barrier recovery, but glycerin replacement through topical administration corrects each of these conditions.
Another action of glycerin is its facilitation of the degradation of desmosomes, the incomplete digestion of which is associated with dry skin.
It is also thought that transepidermal water loss and skin dryness are aggravated by cold and dry weather, due to an extensive, elevated level of skin lipids in the solid state. If this is true, any substance that maintains a higher proportion of lipids in the liquid crystalline state may be an effective moisturizer. Glycerin has been shown to influence the phase transition of stratum corneum lipids, with 10% glycerin inhibitng the transition from liquid to the solid crystalline phase. A more recent study had similar results, showing that glycerin increases the mobility of stratum corneum lipids and proteins, thereby retaining the fluidity of the components of the stratum corneum under dehydrating conditions.
The optimal concentration for glycerin in improving dry skin appears to be 20%, as only small additional benefits were observed beyond this concentration. A separate double-blind study on 17 human volunteers validated these result, showing that 20% glycerin in a cream base increased the hydration of the skin relative to a placebo cream.
Water activity -- the escaping tendency of water in a system and an indicator of how tightly water is bound -- is important in topically applied formulations as exposing the skin with high water activity tends to increase skin hydration and permeability. In the case of glycerin however, it promotes skin hydration even at reduced water activity of the applied topical formulation, attesting to its strong moisturizing effect.
In a pilot study, topical formulations containing 3.75% glycerin + 1.25% hydrolyzed jojoba esters led to significantly lower transepidermal water loss than glycerin alone in a standard base skin care lotion, suggesting that glycerin works in tandem with hydrolyzed jojoba esters to enhance skin moisturization.
It is also important to note that the moisturizing efficiency of glycerin depends on the solvent in which it is dissolved. Glycerin dissolved in water reversed skin dryness induced by treatment with sodium lauryl sulphate, but did not have a moisturizing effect when dissolved in medium chain triglycerides or a mixture of medium chain triglycerides and isopropyl alcohol. This raises questions of the ability of moisturizing oil-in-water emulsions to treat dry skin, as the water evaporates within several minutes.
3.2 Improved barrier repair
There is conflicting evidence on whether glycerin aids the recovery of the barrier function in damaged skin. One study showed faster barrier repair in sites that had been damaged by tape stripping or sodium lauryl sulphate and subsequently treated with glycerol, compared to untreated or base-treated sites. A similar study in 2010 however, found that transepidermal water loss remained high in skin damaged by sodium lauryl sulphate even after glycerin application, indicating that glycerin did not restore the barrier function. Glycerin has no effect on the barrier function of normal skin.
The addition of 10% glycerin to an aqueous solution of sodium dodecyl sulphate (SDS), a well-known skin irritant, hinders the ability of SDS micelles to penetrate and perturb the skin barrier, as glycerol reduces the average radius of the aqueous pores in the skin as well as decreases their density in the stratum corneum by more than 50%.
3.3 Dermatitis treatment
The earliest studies on the effect of glycerin in treating atopic dermatitis were comparative trials. In one, 109 patients were treated for 30 days with 20% glycerin cream, its placebo, or a medicinally authorized cream containing 4% urea + 4% sodium chloride in a randomized, parallel and double-blind fashion. Measurements of transepidermal water loss revealed no difference between glycerin and its placebo, while a lower value was found for urea, and clinical assessments of dryness showed that urea was superior to glycerin. Another randomized double-blind study a year later however had opposite results, finding that 20% glycerin cream was as effective as a cream containing 4% urea + 4% sodium chloride in treating atopic dry skin, with the advantage that adverse skin reactions were experienced significantly less with the glycerin cream.
A third study on 24 patients with atopic dermatitis assessed the effects of a 20% glycerin-based emollient. Although the glycerin cream had a positive influence on the skin, improving the hydration of the stratum corneum and restoring epidermal barrier function, there was no significant difference in the local severity score between the glycerin cream and its vehicle after 4 weeks of twice daily treatment.
A topical glycerin formulation for atopic dermatitis was thought to demonstrate significant treatment effect as evidenced by the restoration of the intercellular lipid lamellae (ICLL) organization of the stratum corneum, however, adding to the pile of conflicting evidence.
3.4 Increased skin smoothness
Glycerin causes topographical changes on the skin surface. It smooths the skin and improves its appearance, with the effect lasting at least 24 hours. Another experiment compared the effects of glycerin and water. Applying excess water resulted in a rapid reduction in skin roughness that was detectable to the naked eye, but disappeared within 30 minutes. Applying glycerin to the skin surface on the other hand boosted and prolonged the smoothing effects of water.
3.5 Changes in mechanical properties
The application of glycerin to the skin changes the hysteresis and perhaps also the distensibility of the skin, with the changes appearing to be relatively long-lasting. These effects may or may not be due to increased skin hydration, as skin hydration has both been shown to be a poor predictor of skin mechanics and to be significantly related to increases in hysteresis and decreases in skin elasticity.
Studies on mice suggest that skin elasticity is mediated at least in part by the presence of glycerin transported via water channels in the skin, particularly aquaporin-3. Mice deficient in aquaporin-3 exhibit reduced skin elasticity, which is corrected by topical or systemic administration of glycerin.
3.6 Wound healing
Glycerin seems to aid the healing of skin lesions in rats, reducing the diameter of the lesions by 16% after 3 days. It is also useful in postoperative care for laser resurfacing, as evidenced by a study on 15 patients whose wounds after laser treatment were dressed with petrolatum on one side of the face and a high glycerin content hydrogel on the other. The hydrogel dressing decreased postoperative morbidity and had a shorter epithelial healing time than the petrolatum dressing.
Another indirect line of evidence linking glycerin to improved wound healing is a study on aquaporin 3-deficient mice. Aquaporin 3 is involved in transporting glycerin, and aquaporin 3-deficient mice show impaired wound healing that is corrected with oral supplementation of glycerin. This has been attributed to the reduced synthesis of stratum corneum lipids resulting from decreased epidermal glycerin content.
4. Side Effects
Glycerin has an anti-irritant effect on the skin. It inhibits the penetration of the surfactant sodium lauryl sulphate (SLS) into the skin, and reduces the severity of irritant reactions to SLS, nonanoic acid in n-propanol (NON), sodium hydroxide and DMSO. It is also a non-sensitizer, though there have been 2 case reports of contact dermatitis resulting from the use of glycerin-containing products.
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